Prior art high-temperature superconductors (HTS), particularly Bi-2223 (Bismuth strontium calcium copper oxide Bi2Sr2Ca2Cu3O10+x) and REBCO (rare-earth barium-copper-oxide) or RE-123 superconductors, are available as a single tape conductor with a maximum width for REBCO of either 10 mm or 12 mm with a critical current at 77 K in self field (s.f.) at the present time of no greater than approximately 500 A.
HTS cables have been developed for applications such as electric power lines, particularly with Bi-2223. Two prior art HTS power transmission cables are shown in FIGS. 1A and 1B. Development of REBCO cables is also underway. These cables target a current level of approximately 10 kA. These cables may include many Bi-2223 or REBCO tapes, each of which is rotated (twisted) along the cable length, as shown in FIGS. 1A and 1B. FIG. 1A depicts a cable formed of a single twisted tape, while FIG. 1B depicts a cable formed of a stack of tapes twisted around a longitudinal axis. REBCO is highly anisotropic: if the tape wide surface is exposed to a magnetic field perpendicular to its surface, the critical current at a given temperature is substantially smaller than if the field is parallel. Since standard cable formations expose the tape wide surface to the highest magnetic field impinging on the cable, such cables cannot be used to wind a magnet generating above 1 T. The anisotropy data of an exemplary REBCO superconductor tape is shown in FIG. 2.
FIG. 2 (prepared by SuperPower, Inc.) shows ratios of critical current with the magnetic field normal (parallel to the REBCO c-axis) to tape surface, Ic (H//c or B//c), to critical current with field parallel to tape surface, Ic, vs. magnetic field [T] plots, at selected temperatures for the exemplary REBCO superconductor tape manufactured by SuperPower, Inc. Note that as the temperature increases, the ratio of critical current with the magnetic field normal (to tape surface precipitately drops with the magnetic field.
Since HTS power transmission cables are readily available, prior art HTS magnets have generally employed coils (solenoidal magnets) of HTS cable. However, as noted above, the current-carrying capacity of power transmission cables are inherently limited due to the geometry of their physical configuration, particularly when employed in a solenoidal magnet configuration. In addition, HTS power transmissions may not be optimal for use in prior art HTS magnets since they are generally not designed to achieve maximum or uniform current density. Therefore, there is a need in the industry to overcome one or more of the above mentioned shortcomings.